Board & Card Games Stack Exchange is a question and answer site for people who like playing board games, designing board games or modifying the rules of existing board games. It only takes a minute to sign up.

1 Answer
1

Yes, they do get the bonus. See the first ruling below the card in Gatherer:

Muraganda Petroglyphs gives a bonus only to creatures that have no rules text at all. This includes true vanilla creatures (such as Grizzly Bears), face-down creatures, many tokens, and creatures that have lost their abilities (due to Ovinize, for example). Any ability of any kind, whether or not the ability functions in the on the battlefield zone, including things like “Cycling (2)” means the creature doesn’t get the bonus.

(emphasis mine)

For example, a morphed card:

702.36c To cast a card using its morph ability, turn it face down. It becomes a 2/2 face-down creature card with no text, no name, no subtypes, and no mana cost.

One could argue that the ability to turn the card face-up is an ability of the 2/2 face-down card, negating the Muraganda Petroglyphs condition, but it's not.

Just to clarify (and make certain myself): Mistform Ultimus and Lhurghoyf won't get the bonus, even though the flavor of their rules texts aren't typically "ability-like" at all, and read more like specifications of their more-complicated-than-standard creature type and power / toughness respectively.
– ArthurApr 23 '19 at 14:06

1

what is weird if a face-down creature loses all abilities it cannot morph but it has no abilities, to begin with
– Neil MeyerApr 23 '19 at 15:49

The trick is that a creature can't just "lose abilities", but it can have a continuous effect that removes abilities. And that continuous effect would still apply and remove the Morph ability if the creature were to turn face up, and that's what prevents you from unmorphing it.
– murgatroid99♦Apr 23 '19 at 17:46